feat(KB-651): add changed files diff viewer for tasks

- Add backend API endpoint to get changed files and diffs for a task worktree
- Add useChangedFiles hook for fetching and managing diff data
- Create ChangedFilesModal component with file list and diff viewer
- Integrate modal into TaskCard with click handler to view changes
- Add frontend API integration and type definitions
- Include unit tests for hook and modal components
- Add changeset for the new feature
- Remove deprecated mission-related test files

packages/dashboard/app/components/Board.tsx

@@ -35,7 +35,7 @@ interface BoardProps {
    * Called when the user clicks the "Subtask" button in the inline create card.
    */
   onSubtaskBreakdown?: (description: string) => void;
-  onOpenFilesForTask?: (taskId: string) => void;
+  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string) => void;
   /** Project context for multi-project mode */
   projectId?: string;
   projectName?: string;